Prior to the introduction of Logging While Drilling (LWD) tools and measurements, analysis of cuttings and mud-gas logging were the primary formation evaluation techniques used during drilling. With the advent of LWD, mud-gas logging lost some of its luster and was viewed as a “low technology” discipline. Recently, however, it has come back in favor; as operators have been able to extract valuable reservoir information that they have not been able to obtain by other relatively inexpensive methods.
The present-day approach to mud-gas logging is fundamentally the same as it has traditionally been: extract and capture a surface sample of gas or hydrocarbon liquid vapor from the returning mud line and analyze the fluid for its composition by means of chromatography, e.g. gas chromatography (GC). The fluid, because of the extraction methods most commonly used, comprises essentially the hydrocarbon components C1 to C5. A well site measurement of the total organic (combustible) gas (TG) was also, in general, available immediately at the well site. Using the history of the circulation rate and the record of the rate of bit penetration, the depth at which the surface sample was acquired could be roughly estimated.
A difference between present-day and past surface analysis techniques has been the introduction of more precise means for determining the composition output by the GC and to extend the scope of the gas analysis to include carbon isotopic analysis for geochemical purposes. Typically, this is done by the use of a mass spectrometer (MS). To this point, this type of analysis has necessitated the use of specialized, bulky equipment and has required access to a suitably equipped laboratory. The turn-around time for a full analysis by a laboratory has been said to be from two to four weeks from the gathering of the sample to the delivery of the final report. (See, for example, Ellis, L, A Brown, M Schoell and A Uchytil: “Mud gas Isotope Logging (MGIL) Assists in Oil and Gas Drilling operations”, Oil and Gas Journal, May 26, 2003, pp 32-41.) With the miniaturization of both GC and MS equipment such analysis is becoming available at the well site, with results available in a matter of hours or less.
The applications claimed for present-day surface mud-gas analysis include at least the following:
1. Identification of productive hydrocarbon bearing intervals, fluid types and fluid contacts;
2. Ability to identify and assess compartmentalization, both vertical and areal;
3. Identification of by-passed/low-resistivity pay;
4. Identification of changes in lithology;
5. The ability to assess the effectiveness of reservoir seals;
6. Identification of the charge history of an accumulation;
7. Determining the thermal maturity of the hydrocarbon identified; and,
8. Geosteering using-gas-while drilling.
The methodology used in going from the simple C1-C5 hydrocarbon component analysis to the capabilities listed above relies on constructing empirically-motivated ratios of combinations of the various hydrocarbon components, plotting these ratios as functions of depth and associating these profiles with the capabilities listed. Examples of these ratios are:
      W    =                                        C            ⁢                                                  ⁢            2                    +                      C            ⁢                                                  ⁢            3                    +                      C            ⁢                                                  ⁢            4                    +                      C            ⁢                                                  ⁢            5                                                C            ⁢                                                  ⁢            1                    +                      C            ⁢                                                  ⁢            2                    +                      C            ⁢                                                  ⁢            3                    +                      C            ⁢                                                  ⁢            4                    +                      C            ⁢                                                  ⁢            5                              =                        Σ          -                      C            ⁢                                                  ⁢            1                          Σ                  B    =                                        C            ⁢                                                  ⁢            1                    +                      C            ⁢                                                  ⁢            2                                                C            ⁢                                                  ⁢            3                    +                      C            ⁢                                                  ⁢            4                    +                      C            ⁢                                                  ⁢            5                              =                                    C            ⁢                                                  ⁢            1                    +                      C            ⁢                                                  ⁢            2                                    Σ          -                      (                                          C                ⁢                                                                  ⁢                1                            +                              C                ⁢                                                                  ⁢                2                                      )                                    C    =                            C          ⁢                                          ⁢          4                +                  C          ⁢                                          ⁢          5                            C        ⁢                                  ⁢        3            where W, B and C are called, respectively, the “wetness”, “balance” and “character” ratios. Other ratios have also been used for both the hydrocarbon species, for example,C1/C3,C2/C3,TG/Σ,(C4+C5)/(C1+C2);the non-hydrocarbon species and combinations of the two.
Notwithstanding advances in equipment, techniques, and turnaround time for surface analysis of mud gas and cuttings, certain drawbacks remain. One problem is depth control; that is, the ability to be able to accurately place the location of an acquired sample. In the presently used method, the depth of the origin of the sample is inferred from the circulation rate and the time between when the sample was extracted at surface and when the bit first passed the sampled depth. Given that pump rates are quite inaccurate and the mud properties vary significantly from surface to bottom hole, the depth determination is often unreliable. Moreover, in general, no allowances are made for the diffusion of the gas within the mud or the inhomogeneity in the mixing as the mud travels along the well bore. This becomes particularly important for thin, stacked reservoirs. As the gas concentration in the mud that reaches the surface is lower than it was originally downhole, highly sensitive instrumentation is needed for the uphole analysis.
A further difficulty is that surface samples tend to be diluted with air and this has to be accounted for in the analysis. Not only do the natural gas “reference samples” against which the extracted sample are compared have to be similarly diluted to obtain reliable results—this requires that the concentration of the mud gas be known a priori—but this dilution makes inaccurate or may even nullify the quantification of non-hydrocarbon gases such as nitrogen, helium and carbon dioxide. This drawback involves, more generally, processes which alter the composition of the gas as it travels to surface and, when applicable, as it travels from wellsite to laboratory. Also, one of the uncertainties that arises when performing mud-gas analysis at the surface is determining the true “background” level of the gas. It is known, for example, that not all the gas may be extracted when the mud is recycled through the mud pits and pumped down the drill pipe. This trace of gas can give a false “background” reading.
To somewhat improve on surface and laboratory analysis of mud gas and cuttings, there has been proposed, for example, downhole analysis for carbon dioxide gas, but with limited capability.
It is among the objects of the present invention to provide techniques which address or solve the aforementioned and other drawbacks of prior art techniques.